It has become common to perform multiple tests in multiple regimes on semiconductor devices on a wafer, as well as at later points in a device's life, including in its final product. Examples of common tests are current-voltage (IV), capacitance-voltage (CV), general radio frequency (RF), and vector network analysis (VNA) tests. Some types of testing, such as CV, RF, and VNA testing benefit from having a control impedance between the test instrumentation and the DUT. Other types of tests however, such as IV tests, do not require such a control impedance. This can be problematic when both types of tests need to be performed.
In IV testing, it is common to use two pairs of triaxial cables (each cable having an outer, intermediate and center connector) between the test instrumentation and two pins at the DUT. At the DUT (distal) end, the center conductors of a first pair of cables are connected to one pin and the center conductors of a second pair of cables is connected to the second pin. The two intermediate conductors of each pair are typically also connected together at the distal end. In operation, the intermediate conductors are typically supplied with a guard voltage that corresponds to the voltage on the respective center conductors. The outer conductors of the triaxial cables are typically connected to a protective ground, since the intermediate and center conductor voltages may be at a high potential.
In CV testing, it is common to use two pairs of two-conductor coaxial cables between the test instrumentation and two pins at the DUT. At the distal end, the center conductors of the first pair of cables are connected to one pin, and the center conductors of the second pair of cables is connected to the second pin. The outer conductors of the cables are typically connected to an instrumentation ground.
AC tests, such as RF and VNA tests, typically require a transmission line between the instrumentation and the DUT. Prior systems use the space between the triaxial cable's center conductor and intermediate conductor as the transmission line for these tests. In order to establish this transmission line, a user must short the intermediate conductors together at the DUT. This short must then be removed before DC tests, such as IV testing, can be performed. It is inconvenient (if not outright burdensome) for the user to change connections at the DUT, particularly when performing a significant amount of both AC and DC testing. In addition, many similar connection systems may be converging into a very restricted space at the DUT, making it even more difficult and time-consuming to change the DUT connections.
Previous four-cable connection systems have been devised to allow a single DUT connection to be used for multiple tests. But no similar system has been devised for two-cable connection systems, even though two-cable systems are more desirable in some situations. Four-cable systems are only necessary when the cable resistance would affect the measurement. When the cable resistance would have a minimal effect on the measurement—for example, when performing high-voltage testing—a two-cable connection system may be more desirable. Two-cable connection systems typically cost less and take up less space than four-cable systems, which can be particularly important when there is limited space at the DUT.
U.S. Pat. No. 7,388,366 describes a four-cable connection system that can use a single connection at the DUT to perform multiple tests. As described in the patent, the four-cable system can also be used to perform two-cable tests. But, this solution requires all four cables to be used, which eliminates the advantages of a two-cable connection system. In addition, many instruments do not natively support four-cable connections. Adapters would allow the four-cable connection system to be used with these instruments, but at the cost of increased expense and complexity. Adapters also lengthen the connection between the instrumentation and the DUT, which reduces performance at higher frequencies.
One solution for two-cable connection systems is to use a switching device that changes connections at the DUT. This allows the instrumentation to use a single cable configuration when connecting to the switching device. But the switching device itself adds cost and complexity to the connection system. Manual switching devices require the user to switch between each set of DUT connections, which adds time between tests. Automated switching devices may use a processor to automatically change the DUT connections. But this adds further complexity and cost, and requires a power source for the processor.
Thus, there is a need for improved two-cable interconnection systems between test instrumentation and devices under test (DUTs).